The invention relates to a high pressure mixing head for the production of a mixture of at least two, preferably chemically reactive, plastic components in a mixing chamber which has inlet openings for the individual components and a common outlet opening for the resulting component mixture. An expulsion piston of the same cross-section as the mixing chamber, and free of any return or reflux grooves, is arranged in the mixing chamber and is adapted for reciprocating movement from a retracted position which leaves the inlet openings to the mixing chamber uncovered, to extended position in which the face of the piston is adjacent the common outlet opening, the inlet opening into the mixing chamber being covered by the piston in this latter position.
In a previously proposed high pressure mixing head (see German Gebrauchsmuster No. 7,006,182), disadvantages are encountered in that on switching over from return flow to injection, undesirably high pressure surges occur. Furthermore, the arrangement employs individual component return ducts or grooves milled directly into the surface of the expulsion piston and these give rise to difficulties as regards satisfactory sealing off of the various plastic components peripherally of the expulsion piston since the later has only a small periphery and since it is necessary to provide a separate return duct groove for each of the plastic components involved.
The disadvantages of that arrangement are avoided in a modified form of mixing head (see German Patent Specification No. 2,145,547) by eliminating the return grooves from the surface of the expulsion plunger and incorporating return ducts in the body or casing of the mixing head. In this arrangement, a separate, auxiliary piston, mechanically connected in parallel with the expulsion piston, is used to control the opening and closing of the return ducts in the mixing head housing. Return grooves are milled in the surface of the auxiliary piston but the return flow passages in the casing of the mixing heads, with which the return grooves communicate in certain positions of the plunger, do not require incorporation of restricted inlet openings, for metering and mixing purposes, similar to those employed in the mixing chamber. As a consequence, a uniform cross-sectional area of return flow path can be provided, thus reducing pressure losses in the system. In this prior arrangement, however, the opening and closing of the inlets to the mixing chamber are accordingly controlled to operate synchronously with the opening and closing of the return ducts by the auxiliary piston. In order to make this possible, it is necessary to provide a relatively complicated design which requires very close manufacturing tolerances. Moreover, there is a particular disadvantage of the system in that the return or recycling of component flow occurs under substantially the same or only slightly reduced high pressure as is maintained during an injection operation. Thus the whole system, including the components of the mixing head, are constantly subjected to such high pressure, something which gives rise to sealing problems and again involves complication of design in order to provide for sufficient strength under the high pressures involved. A further disadvantage is that during the whole of the return, which in practice can be prolonged (for example for 1 hour), the expulsion piston is subjected to the action of the plastics components. More particularly in the case of liquids with very low viscosities the result may be that owing to unavoidable leakages reactions occur between the different plastics components, which can lead to a sticking or corrosion of the expulsion piston. Furthermore so-called calibration which has to be carried out relatively frequently, gives rise to complicated requirements. Such calibration must be carried out in order to provide for precise mutual matching of the quantities of the different plastics components. Calibration is thus carried out in the position of the expulsion piston, in which the inlet openings to the mixing chamber are uncovered, for which reason additional means have to be provided to ensure that the different plastics components cannot simultaneously enter the mixing chamber, as they would otherwise react chemically here.